The recommendation to train outer thighs (more specifically the hip joint abductor muscles) is often heard in the fitness field and rarely in the world of sports. One of the reasons for this is that the hip joint muscles are typically ignored in most sports training programs. The only exception is strengthening the glutes and hamstrings.
However, the hip joint abductor muscles are very important in movement, as for example when running and especially when executing cutting actions or lateral movements. In fact, without strong and explosive abductor muscles it would be impossible to execute quick and explosive changes in direction.
In addition, some of the exercises for the hip joint abductor muscles can strengthen the lateral collateral ligaments which are often torn in sports such as football. I have spoken to many doctors who treat such injuries. They are all in agreement that the hip abductor exercises are perhaps the only exercises that can be used to strengthen these muscles.
The most common recommendation to train outer thighs is to use rubber tubing attached to both ankles. Usually the rubber tubing is relatively short so that you must exert substantial force to pull the tubing apart an appreciable length. As a result, it is difficult to get an ample range of motion.
Much more effective is to use tubing, such as the Active Cords or a low-cable pulley attached to the ankle of the leg to be exercised. They not only allow you to go through a greater range of motion, but execution of the exercise is more sport skill specific.
For most effective execution of this basic hip abduction exercise it is important that the body remain erect at all times as you pull the leg out as far as possible while still keeping the toes pointed directly in front. At the end of the moment you should feel the internal and external obliques contract in addition to the hip abductor muscles.
The standing hip joint abduction exercise is a great beginning exercise to strengthen the abductor muscles. To make the exercise even more sports skill specific, you should do the side lunge. To do this exercise you must use Active Cords so that you can attach the cord to the belt around the hips for resistance.
Execution is also slightly different in that you must initiate movement first with the hips before you step out in the lunge movement. The key is to keep the body erect and facing forward during the execution. When done in this manner you learn more efficient movement when stepping or lunging laterally.
For more information on these exercises see Biomechanics and Kinesiology of Exercise or Explosive Running.
Dr. Yessis can this be done just as effectively with a four-way multi hip machine with a roller and I would ask also the same question for the hip adductor muscles.